The file that contains the Message Filters is called 'msgFilterRules.dat' The 'msgFilterRules.dat' file is located in the mail account folder. * Menu icon > Help > More Troubleshooting Information * Under 'Application Basics' - half way - Profile Folders - click on 'Open Folder' (If Windows), or 'Open directory' (if linux) or 'Show in Finder' (if mac) * Click on 'Mail' folder to see Pop accounts and 'Local folders' * Click on 'ImapMail' to see imap maila ccounts. select relevant account folder and it will contain the 'msgFilterRules.dat' file. Remember to Exit Thunderbird before you try to copy etc The 'msgFilterrules.dat' file can be opened using a text editor program like Notepad.